This invention relates to improvements in an exhaust gas purifying catalyst for promoting oxidation and reduction of hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) in exhaust gas discharged from an internal combustion engine such as a gasoline-fueled or a diesel engine and a combustion device such as a boiler, and more particularly to an exhaust gas purifying catalyst which includes a refractory inorganic carrier coated with a catalytic layer containing noble metals and a NOx trapping substance, and to a method of producing the exhaust gas purifying catalyst, the catalyst being suitably used for reduction of NOx in an oxygen-excess exhaust gas atmosphere.
In recent years, automotive vehicles having good fuel economy have been increasing in view of problems of petroleum resources exhaustion and earth temperature increase. Concerning gasoline-fueled engines used for automotive vehicles, attention has been paid on development of lean-burn engines which are operated mainly on lean air-fuel mixtures. In such lean-burn engines, exhaust gas atmosphere becomes lean (in fuel) to have air-fuel ratio leaner than stoichiometric level during a vehicle cruising in which the engine is operated on lean air-fuel mixtures (i.e., in a lean operating region). It has been known that a NOx reduction effect becomes insufficient under the action of excessive oxygen when a conventional three-way catalyst is used in the lean operating region.
In view of the above, a variety of catalysts have been proposed. One of them is disclosed in Japanese Patent Provisional Publication No. 5-168860, in which platinum (Pt) and lanthanum. (La) are carried on porous carrier so that NOx is trapped in the lean operating region and release NOx in a stoichiometric operating region where the engine is operated on generally stoichiometric air-fuel mixture.
However, even the above catalyst disclosed in Japanese Patent Provisional Publication No. 5-168860 cannot exhibit a sufficient NOx reduction performance, taking a present strict emission standard into consideration.
In order to solve the above problems, researches and developments have been made by the inventors. As a result, the present invention has been completed by controlling locational relationship between platinum and a NOx trapping substance in a catalytic layer formed on a refractory inorganic carrier.
Therefore, an object of the present invention is to provide an improved exhaust gas purifying catalyst and a method of producing the same catalyst, which can effectively overcome drawbacks encountered in conventional similar exhaust gas purifying catalysts can be effectively solved.
Another object of the present invention is to provide an improved exhaust gas purifying catalyst which is excellent in NOx reduction performance and in durability, and an improved method of producing the same catalyst.
A further object of the present invention is to provide an improved exhaust gas purifying catalyst which is high in NOx trapping characteristics in the lean (oxygen-excess) exhaust gas atmosphere and effectively promoting reaction of released NOx and reducing agents such as HC and CO, and a method of producing the same catalyst.
An aspect of the present invention resides in an exhaust gas purifying catalyst which comprises a refractory inorganic carrier. A catalytic layer is coated on the refractory inorganic carrier. The exhaust gas purifying catalyst layer contains platinum and at least one noble metal selected from the group consisting of palladium and rhodium, and a NOx trapping substance. In the catalytic layer, platinum exists close enough to the NOx trapping substance to accept NO2 from the NOx trapping substance which NO2 is produced according to reaction expressed by the following chemical equation: M(NO3)2xe2x86x922NO2+3/2O2+MO where M is the NOx trapping substance.
Another aspect of the present invention resides in an exhaust gas purifying system in combination with a lean-burn engine operated mainly on air-fuel mixture whose air-fuel ratio is leaner than a stoichiometric level, the exhaust gas purifying system comprising. The exhaust gas purifying catalyst includes a refractory inorganic carrier, and a catalytic layer coated on the refractory inorganic carrier. The catalytic layer contains platinum and at least one noble metal selected from the group consisting of palladium and rhodium, and a NOx trapping substance. In the catalytic layer, platinum exists close enough to the NOx trapping substance to accept NO2 from the NOx trapping substance which NO2 is produced according to a reaction expressed by the following chemical equation: M(NO3)2xe2x86x922NO2+3/2O2+MO where M is the NOx trapping substance.
A further aspect of the present invention resides in a method of producing an exhaust gas purifying catalyst. The method comprises (a) preparing a refractory inorganic carrier; (b) forming a first layer part on the refractor inorganic carrier, the first layer part containing at least one of platinum and palladium; (c) forming a second layer part on the first coat layer, the second coat layer containing platinum and rhodium and being higher in concentration of platinum than the first coat layer; and (d) impregnating at least one of the first and second coat layers with NOx trapping substance.